In general, analog cameras must give feedback to a user of what area of a scene to be photographed will be captured when a picture is taken. This has traditionally been accomplished using a built-in device called an optical viewfinder or an optical window that enables the user to visualize the scene and see approximately what area of the scene that will be captured by the camera lens system.
In modern day digital cameras, the viewing of the scene that will be captured is accomplished with either an optical viewfinder or a liquid crystal display (LCD) unit in combination with an optical viewfinder. The LCD in this instance displays the scene on a viewing screen for preview by a user. The viewing of such a scene prior to the image being captured and stored on a removable memory device is called a preview or live view mode of operation. When the image has been captured and stored on such a removable memory device, the LCD unit may also be utilized to view the stored digital image in what is called a post view or review mode of operation where the captured image is retrieved from the removable memory device and displayed on the LCD.
In the live view mode of operation, the scene to be previewed is repeatedly captured by a charged coupled device (CCD) and then buffered to the LCD to allow the image to be refreshed at some predetermined refresh rate or frames per second rate without the benefit of any long term memory storage device. The purpose of this mode of operation is to provide the user with benefit of viewing the exact framing of the image prior to the image being captured. In such a mode of operation, as mentioned earlier, the image is captured at a predetermined frame rate of N frames per second. Thus, for this example, the image is captured at thirty frames per second. Since the camera is capturing the image at a predetermined frame rate, when the camera is re-positioned, the image buffered previously disappears and a new image appears.
Based upon the limitations of the CCD and depending upon the brightness of the scene to be captured, the range of image capture times is limited to a fixed range of between t1 seconds and t2 seconds, where t1>>t2. In short then, to maintain the N frames per second viewing rate, the image capture time may not fall below t2 seconds without causing a substantial degradation in the quality of the buffered image.
In most, if not all digital cameras, the above-mentioned limitations are recognized and thus, such cameras are designed to provide a range of exposure times and aperture settings with a constant gain factor for boosting the signal from the CCD to provide an optimum image at a given brightness. While such an arrangement is generally acceptable for a point, shoot and capture image operation, where the image is captured and stored in a long term storage device, such an arrangement is not optimum for a live view mode of operation because of the wide variable lighting conditions that may be present to the camera as the user moves from an indoor setting to an outdoor setting, and from a bright sunny setting to a relative dark shadow setting. Thus, unlike the point and shoot mode for capturing a scene where a strobe or flash is a convenient way of adjusting for poor lighting conditions, in a buffered image mode or live view mode, a strobe or flash would not be a satisfactory solution.
Therefore it would be highly desirable to have a new and improved digital camera that could provide a user with the ability to correct for image degradation in a live view mode of operation resulting from variable low lighting conditions.